1. Field of the Invention
This invention relates to a wire rope construction with reverse jacketed IWRC (independent wire rope core). More specifically it relates to such construction where the wire rope has no more than 18 outer strands and where the jacket consists of nylon.
2. Description of the Prior Art
Most wire ropes in the wire rope industry are designed so that outer rope strands are laid in the same direction as the strands of the core. For example, if the outer rope strands are laid to the left the same is done with the strands of the core. This is done so as to minimize contact loads between the two. In this manner the core strands do not deteriorate very quickly allowing the rope to fail first primarily from the outside. This allows users to count outer rope strands broken wires and use these as a retirement criteria for the rope. This method of making and inspecting ropes is standard in the industry and is a recognized method to use ropes in a safe manner.
Most of the ropes manufactured as described above will have a tendency to have their ends rotate under load. This is because all the strands of the rope want to straighten under load. Non-rotating ropes are a special category of ropes designed in such a way as to minimize or even prevent completely this rotation. These ropes are usually utilized in crane applications where it is not desirable to have the load rotate during lifting. The lifting end of the rope is always used unrestrained and free to rotate. If a conventional rope is used the rope will unlay, which is also undesirable.
Common designs used for these applications consist of multi strand ropes having the interior core strands laid in a direction which is opposite to the one of the outer rope strands. In these situations both the outer rope strands and the core strands want to unlay under load but they do it in opposite directions. It is a known fact in the industry that the larger the core diameter relative to the individual diameter of the outer rope strands, the better the antirotation properties of the rope. This is because the torque developed by the core can better counteract the torque developed by the outer strands of the rope.
There are three main categories of non-rotating ropes on the market: the 34-35 strand ropes with round and compacted strands; the 18 strand also with round and compacted strands; and finally there is also an eight strand, low cost and lower performance variety consisting of what is commonly known as 8 strand reverse IWRC rope.
The following list identifies these ropes from worst to better in relation to their anti-rotating properties.
Worst performance: 8 strand reverse IWRC rope
Intermediary performance: 18 strand non-rotating rope
Best performance: 34-35 strands non-rotating ropes.
The reason for this behaviour is quite simple: the core in the eight strand rope is the smallest of the three types described above so it does not counteract the torques of the outer strands as well as the larger cores of 18 strand, and particularly 34-35 strands. It should be noted that non-rotating wire ropes with 18 outer strands or less have generally unsatisfactory performance, with the worst cases being ropes of 8 strands or less.
Since the outer strands of these ropes cross-cut at approximately 90xc2x0 angle, the outer strands of their respective cores, they usually exhibit a rapid, invisible core deterioration that cannot be detected from the outside. In other words the detection of outer broken wires cannot be used to assess the inner rope condition. This is particularly the case of 8 strands reverse IWRC ropes and also of 18 strands ropes, while this condition is less severe with the 34-35 strands ropes.
It is hence normal to retire ropes having 18 strands or less from operation after a fixed number of hours or cycles to avoid the xe2x80x9csurprisexe2x80x9d of a sudden internal failure. Another alternative is to jacket the core with plastic materials to prevent the abrasion taking place at the rope strand-core strand interface.
It is already known to provide a jacket of a thermoplastic material, such as polypropylene, around a lubricated core, as disclosed for example in U.S. Pat. No. 4,120,145.
Applicant""s own U.S. Pat. No. 5,386,683 also discloses a jacketed core in which the plastic material of the jacket is identified as polyethylene, polypropylene, nylon or another suitable thermoplastic material.
However, none of the above prior art patents deal specifically with wire ropes of 18 outer strands or less that have reverse jacketed IWRC lay, since the applicant found that with such wire rope construction the commonly employed jacket of polypropylene produces essentially no improvement over the non-jacketed construction and is therefore unsatisfactory.
When reviewing the situation it became obvious that a conventional cushioned core solution and approach did not work in this case. The examination of the polypropylene jacket showed that it had perforated at all the contact points between the outer stands and the core. A conclusion was reached that when dealing, for example, with an 8 strand rope or an 18 strand rope of reverse IWRC lay, the compression load applied by the outer strands on the core would be higher than the compression load applied by the outer strands of a 34-35 strand rope. The same would apply to all such wire ropes of 18 outer strands or less, which must therefore be considered as a special category of non-rotating ropes to which the present invention applies.
The present invention resides in providing a nylon jacket in lieu of polypropylene jacket in wire ropes having at most 18 outer strands and a reverse IWRC lay. Despite the fact that nylon has been mentioned as a suitable jacket material in the past, it was always mentioned as a substitute or alternative material to polypropylene, performing essentially the same function. It is, therefore, surprising and unexpected that in the special category of wire ropes which are under consideration herein, nylon jacketing of the core acts very differently than that of polypropylene, providing essentially double the protection as will be shown later.